1. Technical Field
This invention relates to the technology of cleansing the exhaust from a gasoline-fueled automotive engine, and more particularly to catalysis technology that is operative with exhaust gases from a lean-burn internal combustion engine (excess oxygen providing a redox ratio of 0.03-0.9).
2. Discussion of the Prior Art
Emissions from mobile sources, such as automobiles, that contain NO.sub.x but little excess oxygen are removed by reaction with in situ reductants such as carbon monoxide or hydrocarbons (HC) when passed over catalysts, often containing rhodium. Such catalysts would be ineffective in the presence of a large excess of O.sub.2 (see K.C. Taylor, "Automobile Catalyst Converters", Springer, Berlin, 1984). Recently, copper-exchanged zeolites have been found to reduce NO.sub.x in the presence of excess oxygen (see U.S. Pat. No. 4,934,142 and Japanese patent application publication Hei 3-52644, 3/6/91). Use of such a copper-exchanged zeolite catalyst with a gasoline-fueled engine operated under lean-burn combustion, results in relatively low NO.sub.x conversion, such as 25-35%, and relatively low HC conversion at or substantially below 60%. To increase the conversion of NO.sub.x and HC in the presence of excess oxygen from a gasoline-fueled vehicle, the prior art has conceived injecting hydrocarbons into the emissions ahead of the zeolite. This increases the hydrocarbon content in the exhaust (see Sato et al, Applied Catalysis, Volume 70, L 1-5, 1991). However, it is difficult to meter small doses of such gaseous reductants to accurately match the varying NO.sub.x content of the exhaust gas and to reliably and safely store such gaseous reductants on-board a vehicle. Moreover, injection of additional hydrocarbons does little to improve the overall hydrocarbon/CO conversion efficiency.
It is therefore an object of this invention to provide a catalyst system that operates on the exhaust of a gasoline-fueled lean-burn controlled engine. Under lean conditions, the HC, CO, and NO.sub.x conversions of the invented catalyst exceed the HC, CO, and NO.sub.x conversions of a conventional three-way catalyst.